As is known, in engine belt drives, an endless belt connects the drive shaft to an electric machine and possibly one or more auxiliary members. When using a reversible electric machine, i.e. capable of operating both as a motor and generator, in which case, the belt branches have different tensions, depending on the operating mode of the electric machine, belt tension is controlled using two-arm belt tensioners.
The most commonly used two-arm belt tensioners comprise two belt tensioning arms, which have respective hinge portions mounted to rotate on the same hinge pin about a common axis, and support respective idle pulleys, each acting on a respective branch of the belt. The two arms are loaded towards each other by a normally spiral forcing spring located between the hinge portions of the arms to force the idle pulleys against the respective belt branches and so compensate for variations in belt tension.
Though widely used, known belt tensioners of the type described above have the drawback of being extremely bulky, particularly in directions perpendicular to the hinge axis, and therefore unsuitable for highly compact combustion engines. This is mainly due to the type of forcing spring used, the size of which is determined by the spring having to provide sufficient force to ensure correct tensioning of the belt, while at the same time being flexible enough to cater to continual variations in the tension of the belt branches.